The invention relates to optical fiber devices and methods for tuning the birefringence of waveguides (polarization of optical signals) using such devices and methods. More particularly, the invention relates to all-fiber optical devices, methods and systems for modifying the birefringence in microstructured optical fibers.
Optical devices that modify the properties of optical signals include devices such as modulators, attenuators, and polarization controllers. Such devices use various means to vary refractive properties of one or more regions of the device to change the amplitude/phase of a signal propagating through the device. Conventional devices of this kind use lithium niobate, electroabsorption and/or other configurations to affect the propagation properties (e.g., attenuation, birefringence) of an optical fiber or other waveguide arrangement. Typically, conventional modulators and other devices such as polarization controllers are relatively costly and bulky devices that introduce an undesirable amount of loss. However, all-fiber devices inherently exhibit lower loss and are easier to package than conventional devices.
For example, with birefringent optical fibers (or polarization maintaining fibers), the birefringence is categorized as geometrical birefringence or stress-induced birefringence. Geometrical birefringence is produced by the axially asymmetrical core or structures surrounding the core. Stress induced birefringence is generated by a non-symmetrical stress in the core, where stress applying rods with larger thermal expansion coefficients than silica are introduced in the vicinity of the core. For example, see FIG. 1. However, the configuration of such devices makes them relatively difficult to attain tunability in birefringence.
Accordingly, it is desirable to have available an all-fiber, optical device such as an optical birefringence device that has enhanced tunable birefringence, has easier manufacturability and generally is smaller in size than conventional polarization controllers and modulators, and is relatively easy to splice to conventional fibers.
The invention is embodied in an optical fiber device such as a tunable polarization dependent loss element, a polarization controller or a system for use therein, and a method for making the optical device. According to embodiments of the invention, polarization of light is manipulated in microstructured optical fibers. The ability to control light propagation is achieved by establishing spatial asymmetry within a device such as a microstructured optical fiber, e.g., by filling selected pockets or air-holes formed in the microstructured optical fiber with active/tunable materials. The fiber includes of a core and a surrounding cladding layer. Pockets are introduced in the cladding region and extend, e.g., in the axial direction of the fiber. Active materials are infused in the pockets to change the optical properties of the optical signal. The active materials include, e.g., electro-optic material, magneto-optic material, photo-refractive material and thermo-optic material. Those materials change their intrinsic optical properties such as their refractive index according to external field applied (e.g., temperature). That is, the application of, e.g., temperature, light (optical field) or an electric or magnetic field varies optical properties such as refractive index, loss, scattering, or birefringence of the active material, which, in turn, varies or affects the propagation properties of optical signals in the device.
According to an embodiment of the invention, the optical device includes a tapered region that reduces the diameter of the fiber device but maintains the relative dimensional proportions of the cross-sectional index as in the non-tapered regions. In the tapered region, the mode field is not supported by the doped core and spreads into the cladding region, where it interacts with the active materials. Simultaneously, the tapered region allows the active material to be physically closer to the propagated modes compared to conventional arrangements, thus allowing interaction between the active material and the propagating modes. The tapered region also is designed such that the fiber can be spliced to conventional optical fibers with relatively low insertion loss.
According to an embodiment of the invention, the optical device includes a grating such as a Fiber Bragg grating (FBG) or a long period grating (LPG) written in the photosensitive core of the optical fiber. The FBG or LPG includes periodic perturbations in the index of the core and, like the tapered region, enhances the interaction between the optical signal and the active material. The FBG or LPG couples light from the core mode of the fiber into a mode whose field distribution is spread in the cladding, which is sensitive to the change in the refractive index at the silica-air-holes interface. In this manner, the coupled light interacts with the active material.
To induce birefringence in the optical signal, materials are infused in the airholes in such a way to break the axial symmetry of the fiber. Axial asymmetry is achieved by sealing specific air holes with epoxy, so that only the open air holes are filled with polymer. In this manner, there will be a difference in the propagation constants of the orthogonally polarized modes, which is exhibited as birefringence in the fiber. Control (generating and tuning) of birefringence is achieved by filling selective air-holes with active material and by applying an external field to change the refractive index of the material.
For example, temperature-dependent polymers are infused in one or many holes to provide different birefringence. Such arrangements exploit enhanced tunability because the refractive index of the polymers have relatively large temperature dependences. Birefringence tuning is achieved by changing the index of the material (e.g., thermally in the case of polymers). Also, birefringence tuning is achieved electrically by using liquid crystals, whose refractive index varies electrically, in one or many of the holes.
According to other embodiments of the invention, birefringence tuning is achieved by infusing different materials in different holes (e.g., dn/dt greater than 0 for some holes, dn/dT less than 0 for other holes). Also, birefringence is turned on/off by electrically driving micro-fluids, in one or different holes again, into the waist of the taper or in the region where a LPG is written in the core of the fiber.